(1) Summary of the Invention
The present invention relates to improved discontinuous polyamide fiber and Portland cement containing plastic (fluid) cementitious composites which set to form improved hardened composites. In particular, the present invention relates to plastic and hardened cementitious composites incorporating a mixture of the polyamide fibers and microsilica with the Portland cement to produce a hardened cementitious composite with improved flexural loading characteristics, tensile and compressive strength and ductility, and a method for the production of the composites which is useful on a large commercial scale.
(2) Prior Art
Attempts to incorporate discontinuous polyamide particularly aramide (Kevlar.RTM.) fibers into cement matrices has been reported by Walter, P. L., and Majiumdar, A. J., "Properties of Cement Composites Reinforced with Kevlar Fibers", Journal of Material Science, V. 13, 1979, pp. 1075-1083 and by Konczalski, P., and Piekarski, K., "Tensile Properties of Portland Cement Reinforced with Kevlar Fibers", Journal of Reinforced Plastics and Composites, V. 1, October 1982, pp. 378-384. The absence of fiber dispersant in the cement compositions reported in the above references required the use of non-conventional manufacturing techniques, in particular either (1) spray-up followed by suction to dewater the plastic mixture with only partial success in dispersing fibers by Walter et al or (2) laying of aligned fibers in small specimens by Konczalski et al, which is not practical in large-scale applications and is different from random orientation and uniform dispersion of discontinuous fibers. Without a fiber dispersant, a desirable bond could not be developed between the polyamide fibers and Portland cement paste, and thus the tensile strength and post-peak behavior of the fibrous composite was damaged as can be seen from Walter et al.
The prior art has incorporated various fibers, particularly glass fibers, into cement. Illustrative are U.S. Pat. Nos. 3,380,259; 3,384,522, 3,424,203; 3,489,626 and 3,532,132 to Rubenstein; 3,679,445 to Howe et al; 3,949,144 to Duff; 4,039,345 to Emig et al; 4,310,486 to Cornwell et al; 4,414,262 to Hartmann et al; 4,450,128 to Takeuchi; 4,468,429 to Takeda; Re. 27,061 to Rubenstein and Re. 27,144 to Rubenstein. U.S. Pat. No. 4,310,486 to Cornwell et al describes the use of microsilica and a superplasticizer optionally with glass fibers. No improved results are shown with the fibers.
Japanese Patent No. 74 44099 describes a cementitious composite of various fibers including polyamide fibers prepared using 100-200 mesh serpentine (a naturally occurring hydrous magnesium silicate) as a dispersant. The particles were between 100-200 mesh (147 to 75 micron) which is quite large relative to the diameter of the polyamide fibers. The method is suitable for small specimens.
The problems with incorporating polyamide fibers into cement are related to the deterioration of the fibers in the alkali environment of portland cement paste (especially at higher temperatures) and poor dispersion of the fibers which result in inferior hardened cementitious products. The relatively large particles of Portland cement (compared to the polyamide fiber diameter) also adversely influence the bond between the fibers and portland cement paste. Another problem is to provide a method which is useful on a large commercial scale. The prior art has not solved these problems.